The present invention relates generally to fiber-optic transport systems, and more particularly to methods to optically transport data at a data rate of about 5 Gbps without chromatic dispersion compensation.
Telecommunications carriers and equipment vendors have a common business goal: to increase their optical transport efficiency while using the minimum amount of optical fiber resources. In the last two decades, Synchronized Optical Networks (SONET) in US and Japan, as well as Synchronous Digital Hierarchy (SDH) in the rest of the world, have dominated the optical transport layer with data rate increased from 155 Mbps (OC-3/STM-1), 622 Mbps (OC-12/STM-4) and 2.5 Gbps (OC-48/STM-16) to 10 Gbps (OC-192/STM-64). 40 Gbps (OC-768/STM-256) SONET/SDH data rates are expected to be realized in the near future. When the SONET/SDH standard was created, 5 Gbps was defined as OC-96/STM-32. However, due to economic reasons, equipment vendors and carriers have skipped the OC-96/STM-32 optical transport solution and transitioned directly to OC-192/STM-64 rates. As such, no OC-96/STM-32 optical transport systems have been realized to date. However, a 5 Gbps non-SONET optical transport system may be a viable alternative to the OC-48/STM-16 2.5 Gbps and OC-192/STM-64 10 Gbps SONET optical transport systems.
In the mid 1990s, Dense Wavelength Division Multiplexing (DWDM) arrived to further increase the optical transport efficiency by combining multiple optical signals (at present up to 160 channels) onto one fiber cable, thereby increasing the transport efficiency. Typically, the implementation cost of an optical solution with higher throughput does not increase proportionally with the increase in transport efficiency. Therefore, the per-bit cost decreases with a higher throughput transport system. As the Internet rapidly expands and the amount of data traffic skyrockets, Gigabit Ethernet (GE) channels are increasingly being aggregated into SONET/SDH for long distance transport. As an example, SONET/SDH optical transport systems are increasingly using the OC-192/STM-64 data rates which, as expected, have bandwidth efficiency that is better than OC-48/STM-16 for the transport of multiple GE channels.
A typical metropolitan area (metro) or long haul optical transport application transmits optical signals through a few hundred kilometers, and even through a few thousand kilometers sometimes. When the optical data rate increases above 2.5 Gbits, chromatic dispersion becomes a major concern for the performance of the long distance optical transport. As a result of the chromatic dispersion effect, substantial additional costs for the transport system are incurred above the 2.5 Gbps data rate. The chromatic dispersion effect is caused by the different travel velocities of the various optical signal spectrum components. Chromatic dispersion significantly broadens the signal pulses which severely limits the signal detection capability of the optical receiver.
For example, a standard single mode fiber (SMF) has a chromatic dispersion of 17 ps/nm per kilometer (km) at an optical wavelength of 1550 nm. The spectral width of a chirp-free optical signal is approximately equal to the inverse of the minimum pulse duration, or the equivalent of the data rate. Therefore, for a non-return-to-zero (NRZ) binary signal at 10 Gbps, where the minimum pulse duration is 100 ps, the spectral width is approximately 0.08 nm. A 70-km transmission distance of a 10 Gbps signal in a standard single mode fiber leads to a cumulative dispersion of 1200 ps/nm. Thus, the signal pulse broadens by approximately 100 ps, which is about one bit period. Thus the dispersion-limited transmission distance of a chirp-free 10 Gbps NRZ optical signal is about 70 km in SMF. For a NRZ binary signal at 2.5 Gbps, where the minimum pulse duration is 400 ps, the spectral width is approximately 0.02 nm. Under this condition, the signal pulse broadens by approximately 400 ps due to the cumulative dispersion, the dispersion-limited transmission distance can be reverse-calculated to be 1000 km for a 2.5 Gbps NRZ optical signal. For a NRZ binary signal at 5 Gbps, where the minimum pulse duration is 200 ps, the spectral width is approximately 0.04 nm. After a 300-km transmission of the 5 Gbps signal in a standard single mode fiber, a cumulative dispersion of 5000 ps/nm is incurred. The signal pulse broadens by approximately 200 ps, which is about one bit period. Thus, the dispersion-limited transmission distance of a chirp-free 5 Gbps NRZ optical signal is about 300 km in SMF. In summary, as the optical signal data rate increases from 2.5 Gbps to 5 Gbps to 10 Gbps, the dispersion limited transmission distance decreases from 1000 km to 300 km to 70 km respectively. Dispersion compensating techniques at the higher data rates are needed for longer transmission distances.
For longer distance optical transport, dispersion compensation is required. Dispersion compensating fiber (DCF), which exhibits a negative chromatic dispersion, is the standard method for compensating fiber dispersion. A segment of DCF could be inserted in the transmission line after each fiber span between the multiple stages of optical amplification. The negative dispersion value of the DCF required at each node is equal to the cumulative fiber dispersion at that node. At the end of the total transmission span, the cumulative total dispersion should be an optimal value where the distortion of the signal is minimal. For the linear transmission of chirp-free signals, the cumulative total dispersion optimal value is zero.
Metro applications using the 10 Gbps data rate encounter this dispersion compensation issue. The use of DCF would make the metro area networks both inflexible and expensive. It is very difficult, if not impossible, to uniformly compensate the chromatic dispersion at each node using DCF since there will be several nodes with multiple add/drop channels. The current 2.5 Gbps data rate used for metro applications does not provide sufficient bandwidth efficiency. As previously discussed, the 5 Gbps metro optical transport could support up to 300 km transmission distances without dispersion compensation while increasing bandwidth efficiency. It seems that this data rate is a good compromise for metro optical transport.
When compared to a 2.5 Gbps transport system, the 5 Gbps non-SONET optical transport system is a superior transport system choice to maximize data traffic transport efficiency. Similarly, the 5 Gbps non-SONET optical transport system minimizes the chromatic dispersion issue typically found in any 10 Gbps transport system. Another advantage of a 5 Gbps non-SONET optical transport system over a conventional SONET/SDH transport system is its ability to support two OTU1 channels, each of which could carry two Gigabit Ethernet channels.
Therefore, desirable in the art of fiber-optic data transport systems are improved fiber-optic transport systems for both long haul and metropolitan applications that provide increased data transport efficiency and lower SMF chromatic dispersion, with minimal optical fiber resources and at a lower cost per bit.